Bioimpedance-assisted placement of a medical device

ABSTRACT

A system and method for guiding a catheter or other medical device to a desired target destination within the vasculature of a patient via bioimpedance measurements is disclosed. The target destination in one embodiment includes placement of the catheter such that a distal tip thereof is disposed proximate the heart, e.g., the junction of the right atrium and superior vena cava. In one embodiment the method for guiding the catheter comprises introducing the catheter into a vessel of the patient, the catheter defining a lumen through which fluids can be infused into the vasculature of the patient. The catheter is advanced toward a target destination within the vasculature. A first impedance value based on intravascular detection of at least one electrical property related to a first tissue surface of the vessel is calculated to enable determination of the proximity of a distal end of the catheter to the target destination.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/408,181, filed Oct. 29, 2010, and entitled“Bioimpedance-Assisted Catheter Placement,” which is incorporated hereinby reference in its entirety.

BRIEF SUMMARY

Briefly summarized, embodiments of the present invention are directed toa system and method for guiding a catheter or other medical device to adesired target destination within the vasculature of a patient viabioimpedance measurements. The target destination in one embodimentincludes placement of the catheter such that a distal tip thereof isdisposed proximate the heart, e.g., the junction of the right atrium andsuperior vena cava.

In one embodiment the method for guiding the catheter comprisesintroducing the catheter into a vessel of the patient, the catheterdefining a lumen through which fluids can be infused into thevasculature of the patient. The catheter is advanced toward a targetdestination within the vasculature. A first impedance value based onintravascular detection of at least one electrical property related to afirst tissue surface of the vessel, such as electrical current andvoltage, is calculated to enable determination of the proximity of adistal end of the catheter to the target destination.

These and other features of embodiments of the present invention willbecome more fully apparent from the following description and appendedclaims, or may be learned by the practice of embodiments of theinvention as set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

A more particular description of the present disclosure will be renderedby reference to specific embodiments thereof that are illustrated in theappended drawings. It is appreciated that these drawings depict onlytypical embodiments of the invention and are therefore not to beconsidered limiting of its scope. Example embodiments of the inventionwill be described and explained with additional specificity and detailthrough the use of the accompanying drawings in which:

FIG. 1 is a block diagram of an example system with which embodiments ofthe present invention can be practiced;

FIG. 2 is a simplified view of a patient and a catheter being insertedtherein using the system of FIG. 1;

FIG. 3 is a partial cutaway view of the catheter of FIG. 2 disposed in avessel of a vasculature of the patient according to one embodiment;

FIG. 4 is a simplified schematic of portions of the system of FIG. 1,according to one embodiment;

FIG. 5 is a simplified view of a heart and surrounding vasculature of apatient together with the catheter of FIG. 2;

FIG. 6 is a perspective view of a distal portion of a catheter includinga guiding stylet disposed therein according to one embodiment;

FIG. 7 is a side view of a distal portion of a catheter including adirectional flap according to one embodiment;

FIG. 8 is a side view of a distal portion of a catheter including twodirectional flaps according to one embodiment;

FIGS. 9A and 9B are partial cross sectional side views showing a distalportion of a catheter disposed in a vessel and including a deployablewing according to one embodiment;

FIG. 10 is a partial cross sectional side view showing a catheterdisposed in a vessel and including an electrode pair according to oneembodiment;

FIG. 11 is a cross sectional bottom view of a catheter disposed in avein according to one embodiment;

FIG. 12 is a cross sectional bottom view of a catheter disposed in anartery according to one embodiment; and

FIG. 13 is a partial cutaway view of the catheter of FIG. 2 disposed ina vessel of a vasculature of the patient according to one embodiment.

DETAILED DESCRIPTION OF SELECTED EMBODIMENTS

Reference will now be made to figures wherein like structures will beprovided with like reference designations. It is understood that thedrawings are diagrammatic and schematic representations of exemplaryembodiments of the present invention, and are neither limiting nornecessarily drawn to scale.

For clarity it is to be understood that the word “proximal” refers to adirection relatively closer to a clinician using the device to bedescribed herein, while the word “distal” refers to a directionrelatively further from the clinician. For example, the end of acatheter placed within the body of a patient is considered a distal endof the catheter, while the catheter end remaining outside the body is aproximal end of the catheter. Also, the words “including,” “has,” and“having,” as used herein, including the claims, shall have the samemeaning as the word “comprising.”

Embodiments of the present invention are generally directed to a systemand method for guiding to a desirable anatomic location a medicaldevice, such as a peripherally inserted central catheter (“PICC”) orother catheter. In particular, certain embodiments to be discusseddescribe assisting placement of a catheter or other medical devicewithin the vasculature of the body of a patient such that a distal tipthereof is disposed proximate the heart, e.g., the junction of the rightatrium (“RA”) and superior vena cava (“SVC”). In one embodiment,guidance of a catheter tip to such a location is achieved by usingbio-impedance measurements, which can enhance clinical efficacy andimprove patient safety. Thus, a mapping between body impedance andintravascular anatomic location can be achieved in one embodiment. Notethat the catheters to be described for placement within the patient byway of the systems and methods discussed herein include those definingone or more lumens for the infusion and aspiration of fluids from thevasculature. It should be remembered, however, that other types ofcatheters and medical devices can be placed using the principlesdescribed herein. As such, the discussion to follow should not beconstrued as limiting in any way.

In brief, tissue impedance is a location-specific phenomenon within thepatient vasculature. For example, in the thoracic cavity there ismeasurable tissue impedance difference between the different heartchambers as well as between atrial tissue and adjacent vessels,including the inferior vena cava (“IVC”) and the SVC. Indeed, atrialtissue mainly includes myocardial tissue that exhibits a relatively highelectrical conductivity, and thus a relatively low impedance. Incontrast, vascular tissue, e.g., regions of the vena cava (the IVC andthe SVC), includes mainly smooth muscle cells that are much thinner thanthe atrial tissue and therefore possess a relatively low electricalconductivity, and thus a relatively high impedance. At the junction ofthe RA and SVC, the atrial tissue and vascular tissue meet one anotherand thus define an impedance “border zone” where relatively lowimpedance tissue meets relative high impedance tissue. This region isbut one example where relative differences in impedance are found withinthe patient vasculature.

In accordance with one embodiment, a system is disclosed for enablingsuch impedance variations to be monitored during advancement of acatheter or other medical device within the vasculature of the patientso as to enable positioning of a distal tip of the catheter at a desiredtarget destination. The system in one embodiment includes, among othercomponents, a purpose-specific electrical circuit, processor, anddisplay for monitoring intravascular bioimpedance via electrodesdisposed on a distal portion of the catheter. The system and methodsdescribed herein provide a clinician with guidance to assist indirecting the distal tip of the catheter to the desired targetdestination via feedback of impedance detected by the electrodes duringcatheter advancement through the vasculature. Further, the system can beemployed to confirm the catheter distal tip position after catheteradvancement is complete. Again, note that the catheter positioned by thesystem and methods discussed herein is merely representative of one ofmany different types of catheters or other suitable indwelling medicaldevices.

Reference is first made to FIGS. 1 and 2 which depict various componentsof a catheter placement system (“system”), generally designated at 10,configured in accordance with one example embodiment of the presentinvention. As shown, the system 10 generally includes a console 20,display 30, ultrasound probe 40, sensor 50, and impedance components 60,each of which is described in further detail below.

Note that the particular components to be employed in guiding a cathetervia impedance measurements are shown here in the system 10, which systemalso includes additional catheter insertion and guidance functionality,including a pre-insertion ultrasound-based vessel visualization modalityand a magnetic-based catheter tip guidance modality, as will bediscussed below. This notwithstanding, it is understood that theimpedance-based catheter guidance modality, also discussed below, can beemployed independent and apart from the other catheter insertion andadvancement assistance features of the system 10. Indeed, the system 10may only include an impedance-based catheter guidance modality, in oneembodiment. As such, the present discussion presents merely one exampleof an environment in which embodiments of the present invention can bepracticed.

FIG. 2 shows the general relation of the above-referenced components toa patient 70 during a procedure to place a catheter 72 into the patientvasculature through a skin insertion site 73. FIG. 2 shows that thecatheter 72 generally includes a proximal portion 74 that remainsexterior to the patient and a distal potion 76 that resides within thepatient vasculature after placement is complete. The system 10 isemployed to ultimately position a distal tip 76A of the catheter 72 in adesired position within the patient vasculature. In one embodiment, thedesired position for the catheter distal tip 76A is proximate thepatient's heart, such as in the lower one-third (⅓^(rd)) portion of theSVC. Of course, the system 10 can be employed to place the catheterdistal tip in other locations. The catheter proximal portion 74 furtherincludes a hub 74A that provides fluid communication between the one ormore lumens of the catheter 72 and one or more extension legs 74Bextending proximally from the hub.

The console 20 of FIGS. 1 and 2 can take one of a variety of forms andoptionally houses various system components. A processor 22, includingnon-volatile memory such as EEPROM for instance, is included in theconsole 20 for controlling system function and intravascular impedancecalculations during operation of the system 10, thus acting as a controlprocessor. A digital controller/analog interface 24 is also includedwith the console 20 and is in communication with both the processor 22and other system components to govern interfacing between the ultrasoundprobe 40, sensor 50, the impedance components 60, and other systemcomponents.

In greater detail, the impedance components 60 of the console 20 includemeans for measuring electrical current delivered to electrodes disposedon the catheter 72, as will be described. In the present embodiment, themeans for measuring current includes an ammeter 64 implemented as asampling circuit or other suitable form. Means for measuring voltageacross the electrodes is also included. In the present embodiment, themeans for measuring voltage includes a voltmeter 66 implemented as asampling circuit or other suitable form. Of course, other devices can beemployed to achieve the functionality of the aforementioned means. Aradiofrequency (RF″) or current source 62 is also included for providingan electrical current to the catheter electrodes, as will be described.In addition to these components, other components for enabling impedanceintravascular detection can also be added to the system 10, catheter 72,or both. As shown in FIG. 1, the ammeter 64, the voltmeter 66, and theRF source 62 are operably connected to the processor 22 and ports 52 toenable interoperability therewith. Note that the aforementionedcomponents can be disposed in locations other than the console 20.

As mentioned, the system 10 further includes ports 52 for connection ofconsole components with the sensor 50 and optional components 54including a printer, storage media, keyboard, audio speaker, etc. Theports 52 in one embodiment are USB ports, though other port types or acombination of port types can be used for this and the other interfacesconnections described herein. A power connection 56 is included with theconsole 20 to enable operable connection to an external power supply 58.A battery or other suitable internal power supply 57 can also beemployed, either with or exclusive of the external power supply 58.Power management circuitry 59 is included with the digitalcontroller/analog interface 24 of the console to regulate power use anddistribution.

The display 30 in the present embodiment is integrated into the console20 and is used to display impedance and other information to theclinician during the catheter placement procedure. In anotherembodiment, the display may be separate from the console. As will beseen, the content depicted by the display 30 changes according to whichmode the catheter placement system is in: ultrasound vesselvisualization, magnetic-based catheter guidance, impedance-basedcatheter guidance, etc. In one embodiment, a console button interface 32and buttons included on the ultrasound probe 40 can be used toimmediately call up a desired mode to the display 30 by the clinician toassist in the placement procedure. In one embodiment, information frommultiple modes, such as magnetic and impedance-based catheter guidance,may be displayed simultaneously. Thus, the single display 30 of thesystem console 20 can be employed for ultrasound guidance in accessing apatient's vasculature, magnetic-based guidance during catheteradvancement through the vasculature, and impedance-based guidance and/orconfirmation of catheter distal tip placement with respect to a desiredtarget destination within the vasculature, for instance. In oneembodiment, the display 30 is an LCD device.

The ultrasound probe 40 is employed in connection with the firstmodality mentioned above, i.e., ultrasound (“US”)-based visualization ofa vessel, such as a vein, in preparation for insertion of the catheter72 into the vasculature. Such visualization gives real time ultrasoundguidance for introducing the catheter into the vasculature of thepatient and assists in reducing complications typically associated withsuch introduction, including inadvertent arterial puncture, hematoma,pneumothorax, etc.

The handheld probe 40 includes a head that houses a piezoelectric arrayfor producing ultrasonic pulses and for receiving echoes thereof afterreflection by the patient's body when the head is placed against thepatient's skin proximate the prospective insertion site 73 (FIG. 2). Theprobe 40 further includes a plurality of control buttons, which can beincluded on a button pad. In one embodiment, the modality of the system10 can be controlled by the control buttons, thus eliminating the needfor the clinician to reach out of the sterile field, which isestablished about the patient insertion site prior to catheterplacement, to change modes via use of the console button interface 32(FIG. 1).

As such, in one embodiment a clinician employs the first (US) modalityto determine a suitable insertion site and establish vascular access,such as with a needle or introducer, then with the catheter. Theclinician can then seamlessly switch, via button pushes on the probebutton pad, to another modality, such as magnetic-based orimpedance-based catheter guidance, without having to reach out of thesterile field. These latter modes can then be used to assist inadvancement of the catheter 72 through the vasculature toward anintended target destination.

FIG. 1 shows that the probe 40 further includes button and memorycontroller 42 for governing button and probe operation. The button andmemory controller 42 can include non-volatile memory, such as EEPROM, inone embodiment. The button and memory controller 42 is in operablecommunication with a probe interface 44 of the console 20, whichincludes a piezo input/output component 44A for interfacing with theprobe piezoelectric array and a button and memory input/output component44B for interfacing with the button and memory controller 42. Note thatthe console button interface and probe interface can, in one embodiment,include a touch screen, voice command, or other suitable functionalityto enable ease of system control for the clinician.

The sensor 50 is employed by the system 10 during operation in themagnetic sensing mode to detect a magnetic field produced by magneticelements included in a stylet that is removably received in the lumen ofthe catheter 72. As seen in FIG. 2, the sensor 50 is placed on the chestof the patient during catheter insertion. The sensor 50 is placed on thechest of the patient in a predetermined location, such as through theuse of external body landmarks, to enable the magnetic field of thestylet magnetic elements, disposed in the catheter 72 as describedabove, to be detected during catheter transit through the patientvasculature. The magnetic elements of the stylet magnetic assembly areco-terminal with the distal end 76A of the catheter 72 (FIG. 2) suchthat detection by the sensor 50 of the magnetic field of the magneticelements provides information to the clinician as to the position andorientation of the catheter distal end during its transit within thevasculature.

In greater detail, the sensor 50 is operably connected to the console 20of the system 10 via a cable and one or more of the ports 52, as shownin FIG. 1. Note that other connection schemes between the sensor and thesystem console can also be used without limitation. As just described,the magnetic elements are employed in the stylet 100 to enable theposition of the catheter distal end 76A (FIG. 2) to be observablerelative to the sensor 50 placed on the patient's chest. Detection bythe sensor 50 of the stylet magnetic elements is graphically displayedon the display 30 of the console 20 during magnetic guidance mode.

In this way, a clinician placing the catheter is able to generallydetermine the location and/or orientation (e.g., which way the distaltip 76A of the catheter 72 is pointing) of the catheter distal end 76Awithin the patient vasculature relative to the sensor 50 and detect whencatheter malposition, such as advancement of the catheter along anundesired vein, is occurring. In one embodiment, the magnetic assemblycan be tracked using the teachings of one or more of the following U.S.Pat. Nos. 5,775,322; 5,879,297; 6,129,668; 6,216,028; and 6,263,230. Thecontents of the afore-mentioned U.S. patents are incorporated herein byreference in their entireties. Note again that buttons included oneither the console 20 or the ultrasound probe 40 can be used to controlsystem functionality during ultrasound mode, magnetic-based catheterguidance mode, or impedance-based catheter guidance mode.

Note that the system described herein in one embodiment can includeadditional functionality wherein determination of the proximity of thecatheter distal tip relative to a sino-atrial (“SA”) or other electricalimpulse-emitting node of the heart of the patient can be determined,thus providing enhanced ability to accurately place the catheter distaltip in a desired location proximate the node. Also referred to herein as“ECG” or “ECG-based tip confirmation,” this additional modality of thesystem enables detection of ECG signals originating from the SA node inorder to place the catheter distal tip in a desired location within thepatient vasculature. Note that the ECG modality can be seamlesslycombined with the other modalities of the system as described herein,namely ultrasound, magnetic-based catheter tracking, and impedance-basedtracking to be described further below. Further details regarding thisECG modality and the other modalities described above can be found inU.S. Patent Application Publication No. 2011/0015533, filed Sep. 29,2010, and entitled “Stylets for use with Apparatus for IntravascularPlacement of a Catheter,” which is incorporated herein by reference inits entirety.

FIG. 2 shows that the catheter 72 is operably connected to the sensor 50atop the patient's chest via a tether 78, with the sensor in turnoperably connected to the console 20 and its included components via acable. In this way, the electrode is operably connected to the RF source62, the ammeter 64, the voltmeter 66, the processor 22, the display 30,and the other system components employed during operation thereof.

FIG. 3 shows a distal portion 76 of the catheter 72 disposed in a vessel80 of the patient 70, as inferred in FIG. 2. As shown, the distalportion 76 includes the electrode array 90, including first and secondelectrodes 90A and 90B that are operably connected to conductive wiresor the like longitudinally extending proximally in the catheter wall,for instance, and operably connecting to the tether 78 (FIG. 2) so as tooperably connect the electrodes 90A, 90B of the electrode array 90 withthe RF source 62 (FIG. 1). It is appreciated that the electrode arrayand constituent electrodes can be configured in a variety of ways andthat the shape, number, position, and type of electrodes can vary fromwhat is depicted and described herein. For example, the electrodes canbe included proximate a distal end of a stylet that is removablyreceived within a lumen of the catheter. These and other possibleconfigurations are therefore contemplated.

FIG. 4 depicts a simplified schematic of the various components directlyinvolved in measuring an impedance of the tissue surface 92 of a vesseland the operating relationship to one another, according to oneembodiment. The components include the RF source 62, which provides anRF current to the electrode array 90, including the electrodes 90A and90B that bound either side of the tissue surface 92 under evaluation.The magnitude of the current provided by the RF source 62 can bemeasured by the ammeter 64 and forwarded to the processor 22 of theconsole 20. The magnitude of the voltage difference between the twoelectrodes 90A and 90B across the tissue under evaluation can bemeasured by the voltmeter 66 and forwarded to the processor 22.

With the system 10 and catheter 72 configured as shown in FIGS. 3 and 4,the catheter 72 can be accurately positioned within the patientvasculature by first disposing the catheter within a vessel of thevasculature such that the electrode array 90 proximate the distal tip76A thereof is adjacent to a tissue surface, such as the tissue surface92 of FIG. 3, thus providing electrical communication between theelectrode array 90 and the tissue surface 92 of the vessel 80. In oneembodiment, such electrical communication is achieved by positioning thecatheter 72 within the vessel 80 such that the electrodes 90A, 90Bphysically touch the tissue surface 92 of the vessel 80, as in FIG. 3.An electrical RF current produced by the RF source 62 can then beprovided to the electrode array 90. In one embodiment, the currentincludes a predetermined frequency and is of relatively low power. Thus,with the distal portion 76 of the catheter disposed against a tissuesurface 92, i.e., the inner wall of the vessel 80 in the presentembodiment and as shown in FIG. 3, the RF current is provided to theelectrode array 90 and measured by the ammeter 64. The resultant voltagedifference between the electrodes 90A and 90B across the tissue surface92 is measured by the voltmeter. The magnitudes of the RF current andvoltage are forwarded to the processor 22.

Upon receipt of the current and voltage data from the ammeter 64 andvoltmeter 66 respectively, the processor can calculate the impedance inthe region of the tissue surface under evaluation, also referred toherein as bioimpedance, according to the equation:

Impedance (Z)=Volts (V)/Current (I).   (1)

As such, in the present embodiment, the processor includes suitablecontrol algorithms with embedded software to sample the current andvoltage data (and any other biophysical parameters), in order toautomatically calculate the bioimpedance. The resulting impedance dataas calculated by the processor 22 or other suitable system component canbe depicted on the display 30 for observation by the clinician. Inaddition, audio tones or other suitable signals can be output by thespeaker 54 or other suitable output device so as to provide additionalfeedback to the clinician. For instance, upon reaching the junction ofthe RA and the SVC, an area where a significant change in tissueimpedance is encountered, the display can indicate the detected positionof RA/SVC junction, and the audio speaker 54 can emit a predeterminedaudio tone to indicate the desired anatomic target location.

The above process can be iterated in real time as the catheter distaltip 76A is advanced in the vessel so as to provide real-time updating asto the calculated impedance value according to the present position ofthe distal tip of the catheter 72. For instance, a first impedancecalculation is calculated and displayed for a first location within thevessel of the catheter distal tip, then a second impedance calculationis calculated and displayed for a second distal tip location. Such aprocess can be iteratively performed and the resultant impedance valuescompared so as to enable a clinician to discern when the catheter distaltip is disposed at a desired target location, such as the RA/SVCjunction, for instance.

FIG. 5 shows a heart 96 of a patient, including areas in and proximatethereto of varying impedance, including the SVC 98, the RA/SVC junction100, and the RA 102. The SVC 98, for instance includes a relatively highimpedance, such as about 130-140 ohms, in one example, while the RA/SVCjunction 100 is at a relatively lower impedance of about 118 ohms forinstance. The RA 102 is of even lower impedance, such as about 84 ohmsin one example. Such location-based variations in tissue impedancevalues can be employed by the system 10 to determine the location of theelectrode array 90 of the catheter 72 (FIGS. 1, 3). Further, knowledgeof the distance from the electrode array 90 to the distal tip 76A of thecatheter enables the position of the catheter tip to be determinedwithin the vasculature, thus enabling its precise placement at a desiredtarget location. Note that the above impedance values are for purposesof illustration and should not be considered limiting.

In light of the above, therefore, comparison of subsequent impedancecalculations for successive catheter distal tip locations in the vesselcan indicate proximity to a desired target location. For instance, arelatively small decrease in impedance values between first and secondtissue surfaces can indicate that the electrodes have passed from theSVC 98 to the RA/SVC junction 100 (FIG. 5), while a relatively largerimpedance decrease between the first and second interior surfaces canindicate that the electrodes have passed from the SVC to the RA 102.This or other suitable processes can be expanded to use multipleelectrode arrays, multiples impedance readings, etc. In one embodiment,only a single impedance reading may be necessary to determine thelocation of the catheter distal tip with respect to the RA/SVC or RA,for instance.

As indicated above, measurement of impedance values at a given catheterlocation, followed by movement of the catheter and subsequentmeasurement at the new location, can be iteratively performed so as todetermine when the catheter has been desirably placed, such as proximatethe RA/SVC junction, for instance. It is appreciated that in oneembodiment, the system 10 includes suitable algorithms to calculate,track, store, and display the impedances at the various discretecatheter locations and the impedance change as the catheter is advancedwithin the vasculature. Further the system 10 can include variousfunctionality to depict and display the tracked data in a user-friendlyvisual format for depiction on the display 30, including electroniccircuits for displaying the impedance data and/or other biophysicalparameters in digital and/or analog format. Note that example insertionsites for the catheter into the patient's vasculature include the arm(cephalic vein), neck (jugular vein) and the groin (femoral artery).Other insertion sites can, of course, be used.

In one embodiment, communication ports and software can be included withthe system 10 to enable biophysical parameters sensed and/or employed bythe system, e.g., impedance, current, and voltage, to be exported foruse by other medical equipment, such as clinical vital sign equipment,hemodynamic systems, anesthesia systems, electrophysiology lab systems,computers, storage systems, data analysis systems, etc.

In other embodiments, the electrode array of the system can vary fromwhat is described herein for use in identifying and confirming thespecific anatomic location within the vasculature and proximate theheart, including bipolar and/or monopolar electrodes that are includedwith a catheter, included stylet, or other indwelling medical device.Further, in one embodiment, the impedance values detected by the systemdescribed herein can be used to map the vasculature about the heart,which data can be correlated with radiographically acquired landmarks ofthe patient's anatomy.

As mentioned above, the electrodes 90A, 90B are operably connected tothe console 20 by the tether 78 via the sensor 50, in one embodiment. Inthis case, the tether 78 and/or associated connectors are configured topenetrate through a sterile barrier surrounding a sterile fieldestablished about the patient's catheter insertion site withoutcompromising the sterile field so as to enable the electrodes 90A, 90Bto operably connect with the console 20. Examples of and further detailsregarding such sterile field breaching can be found in U.S. PatentApplication Publication No. 2011/0015533, which is incorporated byreference above.

In a further embodiment, it is appreciated that multiple electrodes orelectrode arrays can be included or associated with the catheter suchthat multiple impedance measurements can be made simultaneously atdiffering locations along the length of the distal portion of thecatheter. In yet another embodiment and as mentioned above, it isappreciated that ECG-based catheter tip location can be used in concertwith the impedance-based location techniques described herein. In such aconfiguration, the ECG-based location method can be used to direct thecatheter distal tip to a generally preferred area, after whichimpedance-based location can be employed to precisely place the catheterdistal tip at a desired location within the vasculature. Further detailsregarding such ECG-based location can be found in U.S. PatentApplication Publication No. 2011/0015533, incorporated by referenceabove.

As mentioned, in one embodiment it is necessary to position the catheter72 within the vessel 80 such that the electrodes 90A, 90B of theelectrode array 90 are in physical contact with the interior tissuesurface 92 of the vessel, as seen in FIG. 3. In one embodiment, this canbe achieved by deviating the distal portion of the catheter within theblood stream of the vessel in which it is disposed. FIG. 6 shows anexample of an apparatus for such a deviation, including a stylet 110disposed within the lumen 72A of the catheter 72 such that a distalportion of the stylet extends distal to the distal tip 76A of thecatheter. The stylet 110 includes a diversion flap 112 pivotably mountedat a distal end of the stylet so as to be able to be selectively movedbetween an aligned position and the deviated positions shown in phantomin FIG. 6. An actuating wire 114 is attached to the flap 112 and extendsthrough the length of the stylet 110 so as to enable a clinicianexternal to the patient to selectively deviate the flap. Deviation ofthe flap from its aligned position of FIG. 6 causes the flap tointerfere with the blood flow through the vessel, which in turn causesthe stylet 110 and the distal portion 76 of the catheter 72 to be pushedto one side of the vessel, thus enabling the electrodes 90A, 90B tophysically contact the interior surface of the vessel. Once physicalcontact of the electrodes 90A, 90B is no longer needed, the actuatingwire 114 can be moved to bring the flap 112 into alignment, thusstopping interfering engagement of the flap with the vessel blood flow.Note that the particular configuration and shape of the flap and styletcan vary from what is shown and described herein. Also, the stylet canbe extended from the catheter distal tip 76A either less or further thanwhat is shown in FIG. 6.

FIG. 7 shows a catheter diversion feature according to anotherembodiment, wherein a diversion flap 122 is included at the distal tip76A of the catheter 72 and has operably connected thereto an actuatingwire 124 for enabling a clinician to selectively fold the flap from thealigned position to the phantom deviated position shown in FIG. 7. Theflap 122 operates in similar fashion to the flap 112 of FIG. 6 incausing deviation of the catheter 72 in the blood stream so that atleast a portion of the electrodes 90A, 90B can contact the vesselinterior surface.

FIG. 8 shows a catheter diversion feature according to anotherembodiment, wherein two diversion flaps 122 are included at the catheterdistal end 76A, each being operably connected to a separate actuatingwire 124 so that deviation of the catheter distal portion is selectivelyachieved by actuating one or both of the flaps in order to deviate thedistal portion in a particular direction. In one embodiment, the flapscan be used in concert to maintain the catheter in a central portion ofthe vessel so as to enable the catheter to be guided past difficult ortortuous vascular anatomy and to reduce vessel wall damage. Note thatthe number, shape, size, and particular design of the flaps disclosedherein can vary from what is shown and described.

FIGS. 9A and 9B show a catheter diversion feature according to anotherembodiment, wherein a distal portion of the catheter 72 includes adeployable flap, or wing 132, for interacting with the vessel blood flowto cause deviation of the distal portion 76 of the catheter toward thewall of the vessel 80 in which the catheter is disposed. In turn, thisenables contact to be made between the interior wall of the vessel 80and the catheter electrodes of the electrode array 90. As shown in FIG.9B, in the present embodiment the wing 114 is triangular orsemi-pyramidal in shape, though in other embodiments other wing shapesare possible. The wing can be selectively extensible/collapsible in oneembodiment.

In one embodiment, it is appreciated that impedance-based guidance andmeasurement within a vasculature can be employed to detect regions ofabnormality within vessels. For instance, an impedance measuringcatheter or other intravascular device employing the methods asdescribed herein can be used to detect plaque locations within coronaryarteries, such as early-stage atherosclerotic lesions including foamcells and fatty deposits within intima. Such plaque deposits areunstable and are prone to rupture, which can expose the subendothelialplaque to blood flow. This in turn can lead to platelet clot formationand unstable angina or acute myocardial infarction. Detection of suchregions via impedance difference measurement with respect to surroundingvessel tissue can enable prophylactic treatment (e.g., angioplasty,stents) to be commenced to alleviate any danger therefrom.

Impedance-based guidance and measurement can also be employed in oneembodiment to detect pre-stenotic lesions in veins and/or arteries. Itis noted that stenosis of atherosclerotic coronary and peripheralarteries, as well as central and peripheral veins (including veinsincluded in an AV access circuit for hemodialysis) is a common problemoften treated with angioplasty. Detection of such regions via impedancedifference measurement (“mapping”) as described herein with respect tosurrounding vessel tissue can enable prophylactic treatment to becommenced to prevent problems in risk areas such as those prone torestenosis and/or de novo stenosis while still in early-stagedevelopment in the vessel wall and prior to significant vesselconstriction. In one embodiment, a solid body catheter or catheterincluding a lumen is employed for carrying the impedance electrodes fordetecting stenotic and/or pre-stenotic lesions. An example configurationis shown in FIG. 10, wherein the catheter 72 including the electrodes90A, 90B is disposed within the vessel 80 such that the electrodes arepositioned adjacent a pre-stenotic lesion 140. Differing impedancemeasurements on and around the lesion 140 can indicate its presence to aclinician, who may then treat the area as needed. In another embodiment,the impedance electrodes are included on an angioplasty balloonassembly, a stent assembly, or a drug-eluting balloon assembly so thatthe appropriate treatment (e.g., angioplasty, stenting, drug delivery)can be administered immediately after detection of the pre-stenosisregion. Note that the detection of stenotic and/or pre-stenotic lesionsmay require, in one embodiment, the use of an RF source frequencydistinct from that for impedance-based catheter tip placement.

In yet another embodiment, it is appreciated that impedance measurementwithin a vessel can be employed to ensure that access to an intended oneof an artery or vein has been achieved. It is appreciated that duringendovascular procedures inadvertent cannulation of a vein instead of anintended artery (or vice versa) can produce adverse effects duringprocedures including cardiac catheterization, central venous catheterplacement, etc. Measurement of impedance values for portions of aninterior wall of a vessel after access thereto is achieved can indicatewhether the vessel is an artery or vein, thus enabling a clinician toconfirm that the proper vessel type has been accessed. It is noted thatimpedance values for arteries generally fall between those of veins andmyocardial tissue. This relationship enables discrimination betweenveins and arteries to be achieved. Thus, arteries, such as thosetypically cannulated during endovascular procedures (femoral,subclavian, brachial, etc.) can be identified by their impedance. Veinscan be similarly identified, thus reducing the potential for adverseevents related to incorrect vessel puncture.

The above impedance relationship is depicted in FIGS. 11 and 12, whereinin FIG. 11 a relatively thick-walled vein vessel 80 having a thicknesst₁ is shown. The catheter 72 including electrodes such as the electrode90B is disposed within the vein vessel 80. Impedance measurements takenby the electrodes can enable the clinician to determine whether thecatheter 72 is disposed within a vein or artery, as described above.FIG. 12 shows a corresponding situation for the catheter 72 disposed inan artery vessel 80 having a thickness t₂ that is thinner relative tothe thickness t₁ of the vein. As mentioned, the measured impedance ofthe artery will be generally lower than that for the vein but higherthan that for heart-related tissue.

FIG. 13 shows that, in one embodiment, extended electrode wires 150 canbe added to one or both of the electrodes 90A, 90B so as to enable theelectrode array 90 to be in operable contact with the tissue surface 92when the catheter 72 itself is not disposed adjacent the surface of thevessel. The extended electrode wires can be configured in a compliantmanner so as to enable the wires to deform as necessary duringadvancement of the catheter 72 through the vasculature yet maintaincontact with the tissue surface 92 so that impedance measurements can betaken when desired. In the present embodiment, four curved electrodewires 150 are attached to each electrode 90A, 90B. Note, however, thatthe number, size, shape, extension, and other configurations of theextended electrode wires can vary from what is shown and describedherein.

Embodiments of the invention may be embodied in other specific formswithout departing from the spirit of the present disclosure. Thedescribed embodiments are to be considered in all respects only asillustrative, not restrictive. The scope of the embodiments is,therefore, indicated by the appended claims rather than by the foregoingdescription. All changes that come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

1. A method for guiding a catheter within a vasculature of a patient,the method comprising: introducing the catheter into a vessel of thepatient, the catheter defining a lumen through which fluids can beinfused into the vasculature of the patient; advancing the cathetertoward a target destination within the vasculature; and calculating atleast a first impedance value based on intravascular detection of atleast one electrical property related to a first tissue surface of thevessel to enable determination of the proximity of a distal end of thecatheter with respect to the target destination.
 2. The method forguiding a catheter as defined in claim 1, wherein calculating the atleast first impedance value is performed by a processor based ondetection of the at least one electrical property by at least oneelectrode pair associated with the catheter, the at least one electrodeoperably connected to the processor, the at least one electricalproperty including a voltage value across the electrode pair, theelectrode pair disposed proximate to the first tissue surface.
 3. Themethod for guiding a catheter as defined in claim 2, wherein calculatingthe at least first impedance value further comprises: providing anelectrical current to an electrode pair disposed proximate to the firsttissue surface; detecting a voltage value across the electrode pair; andcalculating the at least first impedance value based on the providedelectrical current and the detected voltage.
 4. The method for guidingas defined in claim 3, further comprising: establishing physical contactof the electrode pair with the tissue surface before providing theelectrical current.
 5. The method for guiding as defined in claim 4,further comprising: depicting data relating to the calculated at leastone impedance value on a display for observation by a clinician.
 6. Themethod for guiding as defined in claim 5, further comprising: advancingthe catheter so that the electrode pair is proximate a second tissuesurface in the vessel; providing an electrical current to the electrodepair; detecting a voltage value across the electrode pair; andcalculating a second impedance value based on the provided electricalcurrent and the detected voltage.
 7. The method for guiding as definedin claim 6, wherein the target destination is proximate a junction ofthe superior vena cava and the right atrium of the heart of the patient,and wherein an impedance drop between the first impedance value and thesecond impedance value indicates advancement of a distal end of thecatheter toward the junction from the superior vena cava.
 8. A catheterguidance system, comprising: a catheter for disposal within a vessel ofa patient, the catheter defining at least one lumen through which afluid may be infused into the vasculature; an RF source for providing anelectrical current; at least one electrode pair included proximate adistal tip of the catheter to which the electrical current is provided;means for measuring voltage values across the electrode pair when theelectrical current is provided to the electrode pair and the electrodepair is disposed proximate a tissue surface of the vessel; a processorfor calculating impedance values according to the provided current andthe voltage values; and a display for depicting data relating to thecalculated impedance values to assist a clinician in determining alocation of the distal tip of the catheter within the vasculature. 9.The catheter guidance system as defined in claim 8, wherein the meansfor measuring voltage values is a voltmeter, the voltmeter beingoperably connected to the processor, and wherein the system furtherincludes an ammeter for measuring the magnitude of the electricalcurrent, the ammeter being operably connected to the processor.
 10. Thecatheter guidance system as defined in claim 9, wherein the RF source,the voltmeter, the processor, and the display are included in a console.11. The catheter guidance system as defined in claim 8, furtherincluding a magnetic tracking system for magnetically tracking thecatheter within the vasculature and an ultrasound imaging system forultrasonically imaging at least a portion of the vessel.
 12. Thecatheter guidance system as defined in claim 8, wherein the electrodepair is disposed on an outer surface of the catheter proximate thedistal tip thereof.
 13. The catheter guidance system as defined in claim8, wherein the electrode pair is included on a stylet removably receivedin the at least one lumen of the catheter.
 14. The catheter guidancesystem as defined in claim 8, wherein the catheter further includes adiversion component for selectively diverting the electrode pair intophysical contact with the tissue surface of the vessel.
 15. The catheterguidance system as defined in claim 14, wherein a stylet is disposedwithin the at least one lumen of the catheter, and wherein the diversioncomponent includes a movable diversion flap disposed on a distal portionof the stylet extending distal to the distal tip of the catheter. 16.The catheter guidance system as defined in claim 14, wherein thediversion component includes a single movable diversion flap disposedproximate the distal tip of the catheter.
 17. The catheter guidancesystem as defined in claim 14, wherein the diversion component includesfirst and second movable diversion flaps disposed proximate the distaltip of the catheter, each diversion flap operably connected to anactuating wire for selective actuation of the diversion flap by aclinician, the diversion flaps movable to center the distal tip of thecatheter in a center region of the vessel.
 18. The catheter guidancesystem as defined in claim 14, wherein the diversion component includesa deployable flap defining a generally triangular shape.
 19. Thecatheter guidance system as defined in claim 8, wherein the calculatedimpedance values further enable a clinician to determine placement ofthe catheter in one of a vein and an artery.
 20. The catheter guidancesystem as defined in claim 8, wherein the calculated impedance valuesfurther enable a clinician to identify areas in the vessel wherepre-stenoses or early plaque formation are present.
 21. A method forguiding a catheter within a vasculature of a patient, the catheterdefining a lumen through which fluids can be infused into thevasculature of the patient, the catheter including at least a firstelectrode pair disposed on an outer surface thereof, the methodcomprising: introducing the catheter into a vessel of the patient;advancing the catheter toward a target destination within thevasculature; providing an electrical current to the electrode pair withthe electrode pair disposed proximate a first tissue surface;calculating at least a first impedance value according to the electricalcurrent and a voltage value of the electrode pair at the first tissuesurface; and conveying to a user the first impedance value to determineproximity of a distal tip of the catheter to the target destination. 22.The method for guiding as defined in claim 21, further comprising:further advancing the catheter such that the electrode pair is disposedproximate a second tissue surface; providing the electrical current tothe electrode pair with the electrode pair disposed proximate the secondtissue surface; calculating a second impedance value according to theelectrical current and a voltage value of the electrode pair at thesecond tissue surface; and comparing the first impedance value with thesecond impedance value to determine proximity of the distal tip of thecatheter to the target destination.
 23. The method for guiding asdefined in claim 22, further comprising: storing the first and secondimpedance values for future use.
 24. The method for guiding as definedin claim 22, wherein when the second impedance value is smaller relativeto the first impedance value, the clinician can infer the distal tip ofthe catheter is approaching the right atrium of the heart of thepatient.
 25. The method for guiding as defined in claim 22, wherein theelectrode pair physically touches the first tissue surface and thesecond tissue surface when the electrical current is provided.
 26. Themethod for guiding as defined in claim 22, wherein at least oneelectrode of the electrode pair includes at least one extended electrodewire that compliantly engages one of the first tissue surface and thesecond tissue surface.